Laser cavities, in the main, have been made either of specularly reflecting material or, more recently, of diffusely reflecting material. Diffusely reflecting material is preferred in laser cavity applications in order to more uniformly spread the energy from the flash lamp or other exciting means into the lasing material. A diffuse reflecting medium has the advantage of not limiting a laser cavity to a shape with focal points, opening the field to new laser cavity configurations.
The requirement for a highly efficient laser having a cavity made of diffusely reflecting material has resulted in the so called Kigre cavity which is a cavity in which a glass layer is formed over barium sulfate and which generally results in a 97% diffuse reflectivity. It has been found however that such cavities degrade quickly over time and usage, usually by yellowing. Other diffuse cavities have been made of a diffuse gold or silver coating. However, these cavities result only in 94% to 95% diffuse reflectivity.
What will be appreciated is that if the diffuse reflectivity could be increased from 97% to 99% in the visible or near IR region of the electromagnetic spectrum, the overall efficiency of the laser in this region would be increased by as much as 100%. Moreover, if the reflectance of the material in the visible and near IR can be increased, with frequency doublers or similar optical techniques a like advantage can be extended to lasers operating in the ultraviolet region.
However in the past it has been difficult to provide diffuse coatings or cavities made of diffuse material for a number of reasons. The most significant reason is the high energy environment which quickly causes discoloration, pitting, cracking or disintegration. In this regard, laser cavities are frequently subjected to energy in excess of 8 joules per square centimeter.
In a search for suitable materials for diffuse by reflecting laser cavities, up until the present invention, diffusely reflecting polymeric materials have not been employed. It should be noted that diffusely reflecting polymeric materials have been utilized for reflectance standards and coatings as described in U.S. Pat. No. 3,764,364 as well as in companion Patent No. 4,035,085. The above patents call for the utilization of a fluorinated aliphatic long chain addition polymer in pressed powder form or in film form for use as reflectance standards and reflectance coatings particularly in light integrating spheres of spectrophotometers.
By way of further background, an experiment by V. Weidner and J. Hsia reported in the Journal of Applied Optics, Optics News Nov. 1986 pps. 18-20, that powdered polytetrafluoroethylene (PTFE), heretofore used only in a packed powder form as a reflectance standard, could be sintered under pressure to produce a durable material for use in reflectometers. No laser cavity use was contemplated for this material. Note that Weidner and Hsia produced only small 2" diameter wafers unsuitable for laser use. Moreover, the reported material is not suitable for use as laser cavity material because carbon black is added to the granular mixture prior to heating. As will be appreciated, any contaminants within the laser cavity material provide sites for pitting and subsequent disintegration.